Fix bugs and make improvements

- Get clone() working on FreeBSD
- Increase some Python build quotas
- Add more atomic builtins to chibicc
- Fix ASAN poisoning of alloca() memory
- Make MODE= mandatory link path tinier
- Improve the examples folder a little bit
- Start working on some more resource limits
- Make the linenoise auto-complete UI as good as GNU readline
- Update compile.com, avoiding AVX codegen on non-AVX systems
- Make sure empty path to syscalls like opendir raises ENOENT
- Correctly polyfill ENOENT vs. ENOTDIR on the New Technology
- Port bestline's paredit features to //third_party/linenoise
- Remove workarounds for RHEL 5.0 bugs that were fixed in 5.1

libc/intrin/once.h

@@ -0,0 +1,21 @@
+#ifndef COSMOPOLITAN_LIBC_INTRIN_ONCE_H_
+#define COSMOPOLITAN_LIBC_INTRIN_ONCE_H_
+#include "libc/intrin/spinlock.h"
+
+#define _once(x)                       \
+  ({                                   \
+    typeof(x) oncerc;                  \
+    static bool once;                  \
+    static typeof(oncerc) onceresult;  \
+    _Alignas(64) static char oncelock; \
+    _spinlock(&oncelock);              \
+    if (once) {                        \
+      oncerc = onceresult;             \
+    } else {                           \
+      oncerc = onceresult = x;         \
+    }                                  \
+    _spunlock(&oncelock);              \
+    oncerc;                            \
+  })
+
+#endif /* COSMOPOLITAN_LIBC_INTRIN_ONCE_H_ */